Electrodeposition of osmium and baths therefor

ABSTRACT

AN OSMIUM PLATING BATH IS COMPOSE OF A SOLUBLE HEXACHLOROSMATE (IV), FOR EXAMPLE, DIAMMONIUM HEXACHLOROSMATE, ACIDIFIED WITH SULFURIC OR SULFAMIC ACID OR A MIXTURE THEREOF. PLATING IS ACCOMPLISHED FROM SUCH A BATH WHICH MAY ALSO CONTAIN AMMONIUM SULFAMATE AS AN ANODIC DEPOLARIZER, AT A PH BELOW ABOUT 4 AND A CURRENT DENSITY OF FROM ABOUT 1 TO ABOUT 8 AMPERS PER SQUARE DECIMETER.

. 3,692,642 Patented Sept. 19, 1972 3,692,642 ELECTRODEPOSITION OF OSMIUM AND BATHS THEREFOR John Michael Notley, Birmingham, England, and James Barr, Dundas, Ontario, Canada, assignors to The International Nickel Company, Inc., New York, N.Y.

No Drawing. Filed Oct. 23, 1970, Ser. No. 83,680 Claims priority, application Great Britain, Oct. 27, 1969, 52,553/69 Int. Cl. C231) 5 /24 US. Cl. 204-47 Claims ABSCT OF THE DISCLOSURE An osmium plating bath is composed of a soluble hexachlorosmate (IV), for example, diammonium hexachlorosmate, acidified with sulfuric or sulfamic acid or a mixture thereof. Plating is accomplished from such a bath which ma also contain ammonium sulfamate as an anodic depolarizer, at a pH below about 4 and at a current density of from about 1 to about 8 amperes per square decimeter.

The present invention relates to electrodeposition and more particularly, to the electrodeposition of osnnum and baths therefor.

Osmium is a very hard noble metal having good resistance to Wear. Despite its lack of workability it has found application for pen points, phonograph needles, electrical contacts and instrument pivots. Osmium deposits, with their characteristic hardness and resistance to wear, would be quite suitable for coatings for electrical contacts. Osmium deposits in combination with another metal or metal oxide would also be useful as surfaces of dispenser cathodes in certain electronic tubes, e.g., klystrons. In view of such considerations, it would be particularly significant if such deposits could be formed by electrodeposition.

It has been suggested that osmium might be electrodeposited from a fused salt electrolyte containing osmium trichloride (OsCl However, such a bath has the serious disadvantage that a temperature in the neighborhood of about 500 C. or more must be maintained during the plating operation. Such disadvantage would not attend the use of an aqueous electrolyte, if a practical electrolyte were available for deposition of osmium. However, as far as We are aware, there has hitherto been no satisfactory aqueous electrolyte for the electrodeposition of osmium on a commercial scale.

It is an object of this invention to provide a novel plating bath for the electrodeposition of osmium.

It is another object of the present invention to provide a process for the electrodeposition of osmium.

Other objects and advantages will become apparent from the following description.

According to this invention, osmium is electrodeposited from an aqueous acidic solution of a soluble hexachlorosmate (IV), advantageously, the ammonium salt (NHQZOSCL; (diarnmonium hexachlorosmate), or the potassium salt K OsCl The solution of the soluble osmium salt may advantageously be acidified with sulfamic acid (HSO NH or sulfuric acid or a mixture thereof, and ammonium sulfamate may beneficially also be added as an anodic depolarizer. The presence of sulfuric or sulfamic acid or both together with their respective salts is advantageous, as it exerts a buffering action which prevents large changes of pH during the plating. The plating bath preferably contains from 2 to 10 g./l. (grams per liter) of osmium as the compound. At osmium concentrations lower than 2 g./l., deposits are of poor quality. Higher concentrations of osmium up to the limit of solubility of the chlorosmate salt may be used, but there is no practical advantage in doing so. Moreover, losses of osmium from the bath by drag-out are increased, and the capital cost of the bath becomes unnecessarily high.

In order to ensure that the bath is stable it should be acidified to a pH value between about 4 and 0. Bath instability at pH values in excess of 4 is evidenced by decomposition of the hexachlorosmate giving a black deposit of osmium dioxide. A bath pH of from 1 to 2.5 is quite suitable and most advantageously, the pH of the bath is from 1 to 1.5. The ammonium sulfamate may be added in any amount up to saturation, e.g., about 10 g./l., and since it is depleted in the course of plating, replenishment of the ammonium sulfarnate may be necessary when long plating times are employed.

The aqueous acidic solution may also advantageously contain a source of chloride ions, e.g. potassium chloride, which exerts a stabilizing efl ect on the bath by inhibiting hydrolysis of the hexachlorosmate ion and preventing formation of insoluble black osmium dioxide. With the presence of an adequate concentration of chloride ions, the bath remains stable over many days even at temperatures of 70 to C. Accordingly, the presence of chloride ions is highly desirable and a useful concentration range is from 0.05 M (3.7 g./l. as KCl) to 0.5 M (37 g./l. as KCl) although any concentration up to the solubility limit could be employed. Generally speaking, the higher the pH value, the more chloride that is needed. An advantageous concentration range is 0.07 to 0.27 M (5 to 20 g./l. as KCl). Other soluble chlorides can also be used, such as sodium and ammonium chlorides. It is convenient for the source of chloride ions to be in the form of the salt having the same cation as the hexachlorosmate.

As mentioned above, sulfamic or sulfuric acid can be employed to achieve the desired pH range. Many other acids can be employed, e.g., hydrochloric acid although in this case chloride ions are also added and allowance should be made when adding the soluble chloride to achieve an optimum chloride ion concentration.

The plating rate and current efliciency both increase with increase in the temperature of the bath, which should be at least 50 C. and advantageously is at least 65 C. Above about 80 C. evaporation from the bath becomes a problem and appropriate measures may need to be taken, such as continuous replenishement of water to retain substantially constant bath concentration. We find the most advantageous temperature to be about 70 C. A temperature of about C. is the maximum which can be used for most practical purposes.

The bath is used in a single-component cell, preferably with agitation, employing insoluble anodes, suitably of platinum or platinized titanium. The cathode surfaces should be of material not attacked by the solution and thus, if copper is employed, it should be protected, for example, by a flash coating of gold.

The bath is preferably operated at a cathode current density from 1 to 8 a./dm. (amperes per square decimeter). Below 1 a./dm. plating is inconveniently slow, and above 8 a./dm. the efliciency is lower. The current density in most cases need not exceed 4 a./dm. and if it is desired to produce relatively thick coatings, the current density is most advantageously in the range from 1 to 3 a./dm.

In order to give those skilled in the art a better understanding of the invention, the following illustrative examples are given:

EXAMPLE I A bath was prepared containing about 10 g./l. of (NH4)2OSCI8, i.e., about 4.3 g./l. of osmium, about 25 g./l. of sulfamic acid and about 10 g./l. of ammonium sulfamate. The pH of this bath was about 1.15. This bath, operated at 70 C., with a current density of 2 a./ dm. a plating time of about 15 minutes and using a platinum anode produced bright, crack-free deposits having a thickness of about 1 m. (micron) on goldflashed copper discs of 0.05 dm. area. The current efficiency was about 25%.

EXAMPLE II A series of plating baths were prepared containing 10 g./l. of potassium hexachlorosmate and various concentrations of potassium chloride. The baths were acidified by the addition of sulfuric acid initially to a pH value of 0.7 to 1.0 and the pH value was thereafter raised to 1.5 with potassium hydroxide. All solutions were operated at 70 C. with platinum anodes. The plating solution was not heated until the potassium chloride and sulfuric acid had been added, since otherwise, decomposition of the hexachlorosmate would have commenced. Plating was caried out onto gold-flashed copper discs 2.5 cm. in diameter (i.e., 0.1 dm? surface area) using a current of 100 ma., i.e., a current density of 1 a./dm. The results obtained are given in the following table.

Weight Plating oi Os Deposited Current KCl concentration time deposited thickness efficiency (g./l.) (minutes) (mg) (,um.) (percent) It can be seen that substantial deposits of osmium were achieved with the use of 6.25 to 18.8 g./l. of potassium chloride.

Diammonium hexachlorosmate (IV) may be prepared in good yield by heating osmium metal powder in a stream of air to oxidize it to osmium tetroxide and collecting the OsO in a hydrochloric acid solution containing hydrazine hydrochloride. Other reducing agents, e.g., ferrous chloride, may be used in place of the hydrazine hydrochloride. Addition of a saturated solution of ammonium chloride to the resulting deep-brown solution of osmium (IV) chloride gives an immediate redbrown precipitate of the diammonium chlorosmate, which is then washed with ice-cold water and methanol. Osminum tetroxide is poisonous, and appropriate precautions should be taken as will be understood by those skilled in the art. The diammonium hexachlorosmate may also be prepared by other conventional methods. While the invention has been described with some emphasis upon ammonium and potassium salts which have proved advantageous, any soluble hexachlorosmate may be used which contains a cation which is non-platable under the operating conditions, e.g., the sodium salt. Hexachlorosmates other than diammonium herachlorosmate which can be used in the plating baths, e.g., potassium hexachlorosmate and sodium hexachlorosmate, can also be prepared by processes similar to the above-described method for preparing diammonium hexachlorosmate.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be Within the purview and scope of the in vention and appended claims.

We claim:

1. A bath for the electrodeposition of osmium having a pH of from 0 to 4 and containing about 2 to about 10 grams per liter of osmium as a hexachloroosmatc from the group consisting of sodium, potassium and ammonium hexachloroosmate, a chloride from the group consisting of sodium chloride, potassium chloride and ammonium chloride to provide a chloride ion concentration in the bath of about 0.05 to about 0.5 mole per liter with the balance being essentially water.

2. A bath according to claim 1 wherein potassium chloride provides a chloride ion concentration of from 0.05 to 0.5 M.

3. A. bath according to claim 2 wherein the chloride ion concentration is from 0.07 to 0.27 M and the pH value is in the range from 1 to 2.5.

4. A bath according to claim 3 wherein the pH value is in the range from 1 to 1.5.

5. A bath according to claim 1 wherein the pH value is in the range from 1 to 2.5.

6. A bath according to claim 1 wherein the pH value is in the range from 1 to 1.5.

7. A bath as in claim 1 wherein the bath contains a member of the group of sulfuric acid and sulfamic acid to provide the pH of about 0 to 4.

8. A process for the electrodeposition of osmium which comprises cathodically depositing osmium from the plating bath defined in claim 1, wherein the bath is maintained in the temperature range from 50 C. to 100 C. and current is passed through the bath at a current density of from 1 to 8 a./dm.

9. The process of claim 8 wherein the bath contains potassium chloride in an amount sufficient to provide a chloride ion concentration of from 0.07 to 0.27 M.

10. The process of claim 9 wherein the pH of the bath is from 1 to 1.5, the bath temperature is about C. and the cathode current density is from 1 to 3 a./dm.

References Cited J. Llopis et al.: Ana'les de' Fisica y Quirnica, Series B, 63, No. 3, pp. 273-281 (1967).

L. Greenspan: Engelhard Indus. Tech. Bulletin, vol. 10, No. 2, pp. 48-49, September 1969.

A. K. Graham et al.: Plating, pp. 47-49 and 79, January 1949.

GERALD L. KAPLAN, Primary Examiner 

